How to check if some css properties are already set?

I need to know if an object has the "left", "right", "top" or "bottom" properties set from an external css file, or not.

I'm tracing the properties after the object is created and chrome returns the value "auto" if th开发者_StackOverflow社区e properties are not set and that helps me, but firefox does not. Firefox returns some px values (eg. 0 for left).

Do you know a cross-browser solution?